Manufacturers of nonroad diesel engines are expected to meet set emissions regulations. For example, Tier 3 emissions regulations required an approximate 65 percent reduction in particulate matter (“PM”) and a 60 percent reduction in nitrogen oxides (“NOx”) from 1996 levels. As a further example, Interim Tier 4 regulations required a 90 percent reduction in PM along with a 50 percent drop in NOx. Still further, Final Tier 4 regulations, which will be fully implemented by 2015, will take PM and NOx emissions to near-zero levels. Manufacturers of maritime engines are also expected to meet emissions regulations though they vary from the nonroad emissions regulations (e.g., International Maritime Organization regulations).
One technique for reducing NOx involves introducing chemically inert gas into the fresh intake gas for subsequent combustion. By reducing the oxygen concentration of the resulting charge to be combusted, the fuel burns slower and peak combustion temperatures are reduced, thereby lowering the production of NOx. In an internal combustion engine environment, such chemically inert gases are readily abundant in the form of exhaust gas, and one known method for achieving the foregoing result is through the use of an EGR system operable to controllably introduce a recirculated portion of the exhaust gas, from the exhaust manifold, into an intake manifold.
To reduce the NOx levels further, a reductant—such as diesel exhaust fluid (DEF)—is injected into the exhaust stream, where it decomposes into ammonia (NH3). Nitrogen oxides react with the ammonia in a selective catalytic reduction (SCR) canister, which includes an SCR catalyst and ammonia oxidation catalyst (AOC). This reaction forms harmless nitrogen gas and water vapor. From the dual catalyst SCR canister, the exhaust gas flow by a NOx sensor, and out the exhaust outlet to the atmosphere.